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Heat transfer in the mantle is a key process controlling the Earth's dynamics. Upper-mantle mineral phases, especially olivine, have been shown to display highly anisotropic thermal diffusivity at ambient conditions, and seismic anisotropy data show that preferred orientations of olivine induced by deformation are coherent at large scales (>50 km) in the upper mantle. Thus heat transport in the upper mantle should be anisotropic. But the thermal anisotropy of mantle minerals at high temperature and its relationship with deformation have not been well constrained. Here we present petrophysical modelling and laboratory measurements of thermal diffusivity in deformed mantle rocks between temperatures of 290 and 1,250 K that demonstrate that deformation may induce a significant anisotropy of thermal diffusivity in the uppermost mantle. We found that heat transport parallel to the flow direction is up to 30 per cent faster than that normal to the flow plane. Such a strain-induced thermal anisotropy implies that the upper-mantle temperature distribution, rheology and, consequently, its dynamics, will depend on deformation history. In oceans, resistive drag flow would result in lower vertical diffusivities in both the lithosphere and asthenosphere and hence in less effective heat transfer from the convective mantle. In continents, olivine orientations frozen in the lithosphere may induce anisotropic heating above mantle plumes, favouring the reactivation of pre-existing structures.

[1] Thermal diffusivity measurements of seven naturally deformed upper mantle rocks were made as a function of pressure (up to 1 GPa), temperature (up to 1250 K), and the deformation fabric of the samples. For each sample the strain-induced crystal preferred orientations of olivine and pyroxenes were measured, and petrophysical models, based on the thermal diffusivity tensors of the olivine and enstatite crystals, were used to evaluate the three-dimensional distribution of the thermal diffusivity. Both model predictions and measurements show that the anisotropy of thermal diffusivity remains large at the rock scale: 15-28%, depending on the strength of the olivine crystallographic fabric. The direction of maximum thermal diffusivity is parallel to the lineation (flow direction), and the minimum of thermal diffusivity is normal to the foliation plane (flow plane). This anisotropy is preserved at high temperature and pressure. However, measured thermal diffusivities are 20-30% lower than model predictions. This discrepancy between measurements and model predictions cannot be explained by the presence of cracks in the samples because the closure of these void spaces, evaluated through the high-pressure experiments, is found to have a negligible effect on measured thermal diffusivities. Thermal diffusivity for all samples displays a weak linear dependence on pressure of $10% GPa À1 . Thermal diffusivities observed in the high-temperature experiments (1000-1250 K) are compatible with a weak radiative contribution to the total heat diffusion.INDEX TERMS: 5112 Physical Properties of Rocks: Microstructure; 5134 Physical Properties of Rocks: Thermal properties; 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; 8130 Tectonophysics: Heat generation and transport; KEYWORDS: mantle rocks, thermal diffusivity, lattice diffusivity, radiative heat transfer, anisotropy, petrophysical models.Citation: Gibert, B., U. Seipold, A. Tommasi, and D. Mainprice, Thermal diffusivity of upper mantle rocks: Influence of temperature, pressure, and the deformation fabric,

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